Actinic prurigo is an idiopathic photodermatosis seen in American Indians; the face and other sun-exposed areas are usually involved, the patients being sensitive to ultraviolet A light. A modified dominant form of inheritance has been proposed for the disease, but formal segregation analysis has never been performed. Large pedigrees of Cree Indians and Mtis (mixed American Indian and white) of Cree ancestry have been ascertained in which the disease appears to be segregating and detailed data on relationships and age of onset are being collected. These data will undergo segregation analysis to confirm or refute a single gene mode of inheritance.